Vintage chill killer

February 2023 | Scott R. Nesbitt

The simple screw-in design of the Thermostart makes for quick replacement if necessary. Photos courtesy of Scott R. Nesbitt

Cold-weather starting is a challenge for diesel engines. One vintage solution that may be unfamiliar to today’s technicians is a simple device called a “Thermostart.” This winter-starting aid was common until the 1980s and was found on many brands of engine.

Diesels use compressed air to ignite fuel. When the pistons squeeze air into the sealed combustion chamber, the air gets hot enough to ignite the fine droplets of injected fuel — except when everything is cold.

Regardless of ambient temperature, the first start of the day is the hardest. To achieve ignition, the compression stroke must be fast, so the air’s heat doesn’t dissipate into the engine’s metal. Compression relief helps, letting the starter get the engine spinning, so inertia aids the starter motor.

Drop the temperature, and the battery produces less power, when more power is needed to overcome the thicker cold oil in the engine. The engine is sucking in cold outside air, which requires even more forceful compression to hit the magic ignition temperature.

Enter the Thermostart. It preheats the incoming air by lighting a fire in the intake manifold.

Inside the device is an electric heating coil. When power is connected, the coil heats up in a few seconds. The heat opens a valve that lets a trickle of fuel hit the coil. It starts burning. After 10 to 15 seconds, it’s time to crank the engine and suck in that hot air (and sometimes actual flame). Repeat the process until the engine starts or the battery poops out.

A heating coil component of a thermostart

Modern diesels have a glow plug in each cylinder. This electrically heated stick ignites the fuel. Just turn the key, and the glow plug heats up. But glow plugs can have their lives shortened by all the pounding, heating and cooling that comes from being in the combustion chamber whenever the engine is running.

The Thermostart requires a supply of fuel, and it complicates the design and operation. But until needed, the Thermostart sits quietly in the intake manifold. It can last for decades. If it fails, replacement is an easy external job. New units cost about $25 online, and virtually all major diesel engine makers used the same basic unit. The Thermostart is easier on cold batteries, as there’s only one heating coil compared to the power needed for multiple glow plugs.

A pencil points to the “Thermostart” intake air heater mounted in the intake manifold of a Yanmar diesel that is also found in John Deere compact tractors. Note the fuel supply reservoir mounted above the device.

To see what this interesting heater looks like in action, search YouTube for “How the flame heater works on a Perkins diesel” and browse through the several videos that come up in a search for “Thermostart.”

If there’s time, check to see if your diesel includes a Thermostart. If you find one, pull it out and check that the heating coil and fuel valve are working, especially if cold weather looms.

Scott R. Nesbitt is a freelance writer and former GCSAA staff member. He lives in Cleveland, Ga.